Hi, I’m seeing the same issue, but it works when explicitly installing ghc@8.6. Looking at haskell-build-system all Haskell libraries are currently built with version 8.6, whereas the newest GHC version available (and thus installed by `guix install`) is 8.8.
I feel that either xmonad should depend on the correct GHC or all GHC versions not used to build libraries should be hidden to avoid this situation. Lars
